This invention generally relates to the field of illumination circuits and, more particularly, to illumination circuits that illuminate EL devices.
Because of their compact size and low current consumption, EL devices are widely used in small battery operated electronic devices. For example, such lamps are used for backlighting liquid crystal displays in portable communication devices, such as cellular telephones. Essentially, an EL device is a capacitor with a phosphorous dielectric. The lamp emits light when it is excited by applying a sufficiently high AC voltage across its electrodes. To emit light continuously, the lamp must be charged and discharged at a low frequency during successive charge and discharge cycles. For this reason, the drive signal for the EL device is a high-voltage, low-frequency AC drive signal. Depending on the size of the EL device and the desired illumination intensity, this signal can have a voltage level in the range of 100-150 volts and a frequency in the range of 100-400 Hz.
In battery operated devices, a battery supplies a DC supply voltage in the range of 1-5 volts that powers a device in which the EL device is used. Such a voltage is significantly lower than the voltage level required to illuminate the lamp. Therefore, an electronic device that uses EL device typically includes an inverter circuit which converts the low DC supply voltage to a high-voltage, low-frequency AC drive signal. Various types of inverter circuits have been used in the past to convert a DC supply voltage to an AC drive signal, including transformers and bridge inverter circuits.
Another conventional inverter circuit uses a pump circuit in which the energy stored in an inductor is switched at high speed to produce a series of high voltage pulses. These pulses successively charge the EL device to a sufficiently high voltage level during a charge cycle. One of the advantages offered by such an inverter circuit is that by increasing the switching frequency, the size of the inductor can be reduced, thereby reducing the size of the inverter circuit. During a discharge cycle, the energy stored in the EL device is discharged to ground through a short circuit. In a pending patent application titled "An Inverter Circuit For Illuminating An Electroluminescent Lamp", which is hereby incorporated by reference, the Applicant of the present invention has disclosed presenting a damping circuit on the discharge path of the EL device in order to reduce humming noise. By closing and opening a discharge switch at a low frequency, the inverter pump circuit produces a low-frequency AC drive signal across the EL device.
In battery operated devices, the battery life is of utmost importance. For this reason, various techniques are being devised to minimize battery current drain at various device circuits, including EL inverter circuit. Therefore, there exists a need for an EL inverter circuit that preserves battery power.